The present invention generally relates to a drying furnace and, more particularly, to a drying furnace used on an automobile body painting line for drying successively painted automobile bodies.
In a known automobile body painting line, automobile bodies are successively passed by means of a conveyor system through a plurality of work stations such as, for example, undercoating and overcoating stations where one or both of electro-deposition coating apparatus and electrostatic coating apparatus are utilized. At the final stage of the painting line, the automobile bodies are allowed to pass successively through a drying furnace within which they are subjected to a baking process for baking the coating to give a baked-finish.
Hitherto, the drying furnace referred to above is available in two types; a direct drying furnace in which blasts of hot gases such as combustion gases are applied direct to the automobile bodies to bake the coating, and an indirect drying furnace in which the coating is baked by heat emanating from radiators having combustion gases flowing therethrough. The direct drying furnace is generally recognized as having numerous advantages, namely, lightness and compactness, a prolonged service life, a quick heating capability to elevate the furnace temperature in a relatively short time, ease to control the furnace temperature in a relatively wide range and with precision, and a high thermal efficiency, but also has a disadvantage in that foreign matter such as dust carried by the hot gases tends to adhere to the coating on the automobile body as the latter is traversed through the direct drying furnace.
On the other hand, with the indirect drying furnace, the possibility of foreign matter adhering to the coating on the automobile body is minimized as compared with the direct drying one and, however, it is not satisfactory as to the service life and the thermal efficiency as compared with the direct drying furnace.
In view of the foregoing, a single drying furnace having two functions of direct and indirect drying furnaces has been proposed such as disclosed in, for example, Japanese Laid-open Patent Publication No. 57-167766 published Oct. 15, 1982, and makes use of the respective advantages of the direct and indirect drying furnaces. This prior art combination drying furnace has indirect and direct drying zones on entry and exit sides, respectively, with respect to the direction of transportation of the automobile bodies through the drying furnace and may be considered a version in which indirect and direct drying furnaces are series-connected together using a single housing structure.
With the prior art drying furnace such as disclosed in the above mentioned publication, each of the automobile bodies which have been painted is first allowed to pass the indirect heating zone where the paint coating is dried to such an extent that the foreign matter will hardly adhere to the surface of the paint coating and is then allowed to pass the direct drying zone for the baking finish. Accordingly, it appears that the combination drying furnace is effective to minimize the adherence of foreign matter to the paint coating during the drying and also to minimize the loss of heat from the furnace.
Apart from the above, the space occupied by either one of the indirect and direct drying zones relative to the length of the furnace is dependent on the speed at which the painted automobile bodies are sucessively transported through the furnace. The speed of transportation of each automobile body in turn varies with the number of automobiles being produced per hour. In view of this, in the prior art combination drying furnace, the ratio of the space occupied by the indirect drying zone relative to that occupied by the direct drying furnace is chosen so as to be of such a value that, even at the maximum available speed of transportation of the respective automobile body, the paint coating can be dried within the indirect drying furnace sufficiently enough to avoid the possible adherence of foreign matter on the automobile body. In other words, in order for the paint coating on the automobile body to be dried sufficiently within the indirect drying zone even though the automobile body is transported at a maximum available speed, the indirect drying zone must extend, and occupy, a substantial distance within the furnace.
Therefore, in using the prior art combination drying furnace, and when it comes to the employment of a relatively low speed of transportation of the automobile body through the furnace, or when the transportation speed is slowed down by and for some reason, the automobile body must travel an unnecessarily great distance through the indirect drying zone where the thermal efficiency is generally lower than that in the direct drying zone. This means that, considering the thermal efficiency of the entire furnace, the combination drying furnace still has a problem to be improved.